Two-step method of making a color picture tube

ABSTRACT

In manufacturing a color picture tube, a plurality of color phosphor dots are placed on a face plate of the tube. A shadow mask used with the tube is made with a plurality of apertures arranged in a predetermined pattern. Each of these apertures have a predetermined size. The predetermined size of each aperture is reduced to a smaller size when the mask is used in placing the phosphor dots on the face plate. The size of each of the apertures is returned to its original, predetermined size when the mask is used for operating the color tube. This disclosure teaches an improvement in the method of reducing and, thereafter, returning the size of the apertures to the predetermined size. The improvement includes a two-step application of powder coating compositions to the shadow mask to reduce the size of the apertures. The first step in the method partially reduces the apertures and the second step in the method reduces the apertures to a final size needed for laying down the phosphor dot pattern on the face plate. Since a powder coating composition is used, the materials may be easily removed from the mask by utilization of a solvent for the composition.

Unite States Patent 1191 Labana et a1.

[ May 21, 1974 1 1 TWO-STEP METHOD OF MAKING A COLOR PICTURE TUBE [73]Assignee: Ford Motor Company, Dearborn,

Mich.

22 Filed: May 17, 1972 21 Appl. No.: 254,231

[52] US. Cl 117/17, 96/36.1, 117/1.7.5, 117/21, 117/29, 117/33.5 CM,117/99,

313/85 R, 313/85 S, 313/92 B [51] Int. Cl. H0lj 9/20, B44d 1/094 [58]Field of Search 117/17, 17.5, 21, 33.5 CM, 117/29, 99; 313/85 R, 85 S,92 B; 96/361 [56] References Cited UNITED STATES PATENTS 3,070,44112/1962 Schwartz ..96/36.1 3,653,901 4/1972 Etter 117/33.5 CM

Lerner 96/361 Lange 117/33.5 CM

Primary E.raminer'Michae1 Sofocleous Attorney, Agent, or FirmWilliam E.Johnson; Keith L. Zerschling [5 7 ABSTRACT In manufacturing a colorpicture tube, a plurality of color phosphor dots are placed on a faceplate of the tube. A shadow mask used with the tube is made with aplurality of apertures arranged in a predetermined pattern. Each ofthese apertures have a predetermined size. The predetermined size ofeach aperture is reduced to a smaller size when the mask is used inplacing the phosphor dots on the face plate. The size of each of theapertures is returned to its original, predetermined size when the maskis used for operating the color tube. This disclosure teaches animprovement in the method of reducing and, thereafter, returning thesize of the apertures to the predetermined size. The improvementincludes a two-step application of powder coating compositions to theshadow mask to reduce the size of the apertures. The first step in themethod partially reduces the apertures and the second step in the methodreduces the apertures to a final size needed for laying down thephosphor dot pattern on the face plate. Since a powder coatingcomposition is used, the materials may be easily removed from the maskby utilization of a solvent for the composition.

10 Claims, No Drawings TWO-STEP METHOD OF MAKING A COLOR PICTURE TUBEBACKGROUND OF THE INVENTION Color television tubes are prepared bydepositing a plurality of groups of color phosphor dots on a glass faceplate of the tube. The size and location of the phosphor dots on theface plate are controlled by interposing a steel shadow mask having aplurality of apertures therein between a light source used in layingdown the dots and the face plate.

The same shadow mask is also used to direct electron beams to illuminatethe dots during operation of the tube.

The best colors and brightest shades are obtained from a tube whendifferent phosphordots are separated from each other by dark areas. Ashadow mask with aperturesof9 to 10 mils diameter is needed fordepositing such well separated dots. During operation of the tube,however, the size of the shadow masks apertures must have a diameter inthe range of 14 to 16 mils for transmission of a larger fraction ofelectron beam energy to illuminate fully the phosphor dots. The shadowmasks apertures, therefore, are smaller in'size during deposition of thephosphor dots than for operation of the tube. The prior art has solvedthis problem in many ways which are disclosed in such U.S. patents asU.S. Pat. Nos.: 3,231,280 to Law, 3,574,013 to Frantzen, 3,604,081 toMoegenbier, and 3,616,732 to Rucinski.

The invention disclosed herein is an improvement in the method ofreducing from a predetermined size the size of apertures of a shadowmask for deposition of color dots and for enlarging such apertures tothe predetermined size for transmission of electron beams forilluminating the dots. The improved method employs a dual application ofpowder coating compositions to reduce the size of the shadow maskapertures in two steps. Since powder coating compositions are employed,they may be removed easily by solvents for the compositions after themask has been used to deposit the color dot pattern on the face plate ofthe tube.

SUMMARY OF THE INVENTION This invention relates to a method ofmanufacturing a color picture tube and, more particularly, to animprovement in the method of manufacturing a color picture tube whereinpowder coating compositions are employed in reducing the size of ashadow masks apertures so that the mask can be used in placing the colordots on the tube.

In the art of manufacturing a color picture tube having a plurality ofgroups of phosphor color clots placed on a face plate thereof, ametallic shadow mask is made with a plurality of apertures arranged in apredetermined pattern. Each of the apertures of the shadow mask has apredetermined size larger than the size of each of the dots to be laiddown on the face plate of the tube. The predetermined size of theseapertures is reduced to a smaller size and the phosphor color dots areplaced on the face plate with the aid of the shadow mask having thesmaller apertures therein. The size of the apertures of the shadow maskare returned to the predetermined size so that the same shadow mask canbe used in operating the tube.

The method of this invention sets forth an improvement in reducing thesize of the apertures and, thereafter, returning the size of theapertures to the predetermined size. The improvement includes thefollowing steps. A first powder coating composition is applied to theshadow mask having apertures of the predetermined size in order toreduce partially the size of the apertures. The first powder coatingcomposition has a molecular weight (M in the range of 10,000 to 100,000and an average particle size below 30 microns.

Once the first powder coating composition is applied to the shadow mask,the mask is heated with the composition thereon to a temperature and fora time sufficient to sinter the composition together but insufficient toinduce any substantial flow thereof. A second powder coating compositionis applied to the first composition on the shadow mask. The secondcomposition has a molecular weight (M,,) in the range of 1,800 to 6,000.The second composition also has an average particle size below 30microns. After the second composition is applied to the firstcomposition on the mask, the mask is heated to a temperature and for atime sufficient to sinter the second composition together and to smooththe edges of the apertures but insufficient to induce any substantialflow of the first composition. The application of the two compositionsis effective to reduce the size of the apertures to that required forlying down the color dot pattern. After the color dot pattern is' laiddown, the two compositions are removed from the mask and the mask isused in the tubes final construction for directing the electron beamsagainst the dot pattern. The powder coating compositions can be appliedto the mask by any known techniques used to coat metal by powdercoatings. These techniques include electrostatic spray, fluidized bed,electrostaticfluidized bed, etc.

ln greater detail, thermoplastic and thermosetting polymeric materialswhich may be employed as either the first or the second powder coatingcomposition include, but are not limited to, polystyrene,polymethylmethacrylate, epoxy resins, cellulose acetate butyrate andcopolymers of glycidyl acrylate or glycidyl methacrylate. To thesepolymeric materials may be added such materials as carbon black whichserves as an antistatic agent and also to absorb stray light during dotplacement on the face plate, antistatic agents which aid in thedeposition of the powder on the shadow mask and thixotropic agents ifdesired.

Since the powder coating compositions applied to the shadow mask areheated to a temperature and for a time sufficient only to sinter theparticles of the compositions together, the compositions may be easilyremoved from the mask by common solvents. Thus, the predetermined sizeof the apertures may be reobtained with little diff culty after thecolor dot pattern has been placed on the face plate of the tube. Moreparticularly, since the powder compositions do not substantiallycrosslink because of the limited heating, the compositions remainsoluble in most common solvents.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention relates to amethod of reducing the size of apertures of a shadow mask which isemployed 1 in forming a color television tube. As is already well 16mils. As is also well known in the art, the diameter of these aperturesis reduced to 9 to 10 mils so that the shadow mask may be used in thedeposition of color phosphor dots on a face plate of the tube. As isalso known, each shadow mask is unique for a color tube in that it isutilized with the small apertures for laying down the color dot patternon the face plate and with the larger apertures for use in directingelectron beams toward dots on the face plate.

The improved method of this invention will be illus trated by aplurality of examples.

EXAMPLE 1 One hundred parts by weight of polymethylmethacrylate (M,,)45,000) and three parts carbon black are mixed together in a twin shelltumbling mixer for 10 minutes. The mixture is extruded through acompounding extruder at a temperature of 180C. The extrudate ispulverized in a fluid energy mill so that is passes through a 270 meshscreen. The powder coating composition so produced has an averageparticle size of 15 microns and constitutes a first powder coatingcomposition.

The first composition is sprayed onto a shadow mask, which'mask has aplurality of apertures aligned in a predetermined pattern. The diameterof these apertures is in the range of 14 to 16 mils. The firstcomposition is applied by an electrostatic spray gun operating at 60 KV.The coated shadow mask is baked at 180C for 3 minutes. The size of themasks apertures is reduced to the range of 10 to l 1 mils without any ofthe apertures being plugged and with the apertures having a slightirregular shape and some rough edges.

A second powder coating composition is prepared by mixing 100 partsbyweight epoxy resin, in this case,

, Epon l004, a trademark of the Shel l Chemical Company lnc., whosechemical formula is wherein n and havinga molecular weight of 1,800 withthree parts carbon black. The mixture is extruded through a compoundingextruder at a temperature of 130C. The extrudate is pulverized in afluid energy mill so that it passes through a 270 mesh screen. Thepowder coating composition so produced has an average particle size ofmicrons and constitutes a second powder coating composition.

The second composition is sprayed onto the first composition already onthe mask by an electrostatic spray gun operating at 60 KV. The coatedmask is baked at 90C for 3 minutes. The size of the masks apertures isreduced to the range of 9 to 10 mils without any of the apertures beingplugged and with the apertures having uniform, circular and smoothedges.

The shadow mask prepared by using powder compositions in the two-stepmethod is utilized to lay down the groups of three different color dotphosphors on a face plate of a TV tube in a normal manner. After thecolor dot pattern is laid down, the two powder compositions are removedfrom the shadow mask by washing 1 the mask in toluene. The cleanedshadow mask is assembled with the tube by a plurality of manufacturingsteps known in the art. The color tube is operated by receiving a colorsignal and the signal so derived is employed to excite the phosphorcolor dot pattern on the face plate. A color picture of acceptablequality is received on the color tube.

EXAMPLE 2 A mixture containing parts by weight of polymethylmethacrylate(Kl, 30,000), two parts carbon black and one part of an antistaticagent, in this case, stearamidopropyl-dimethyl-beta-hydroxyethylammoniumnitrate are mixed by shaking in a plastic jar. The materials are millrolled at 180C for 20 minutes. The mixture is cooled and pulverized topass through a 270 mesh screen. The mixture which passes through themesh screen has an average particle size of about 10 microns andconstitutes a first powder coating composition.

The first composition is sprayed onto a shadow mask by an electrostaticpowder spray gun as described in Example 1 at an operating voltage of 40KV. The.coated mask is baked at C for 3 minutes to sinter the firstcomposition together. This baking is insufficient to induce anysubstantial flow in the composition. The size of the apertures in theshadow mask are reduced to a range of 10 to l 1 mils with slightlyirregular shape and rough edges but with no plugging thereof.

A second powder coating composition is formulated by mixing 100 parts byweight of epoxy resin, in this case Epon 1007 which has a molecularweight of 4,500

and a structural formula identical to Epon 1004, with the exception thatn is 14, with three parts carbon black. The materials are mixed byshaking in a plastic jar. The materials are mill rolled at 140C for 20minutes. The mixture is cooled and pulverized to pass through a 270 meshscreen. This mixture which passes through the screen has an averageparticle size of about 10 microns and constitutes the second powdercoating composition.

The second composition is sprayed onto the mask having the firstcomposition thereon by means of an electrostatic powder spray gun asdescribed in Example 1 at an operating voltage of 40 KV. The coated maskis baked at 120C for 3 minutes to sinter the second compositiontogether. The size of the apertures in the EXAMPLE 3 A mixturecontaining 100 parts by weight of polymethylmethacrylate (R1,, 45,000)and three parts carbon black is prepared by the same precedure as desriad Exama ar t .p sv ati of the first powder coating composition. Thecomposition so produced has an average particle size of about 25 micronsand is applied to a shadow mask as described in Example 1. The coatedmask is baked at a temperature of C for 3 minutes and the size of theapertures are reduced to 10 to 11 mils without any apertures beingplugged.

A second powder coating composition is prepared by mixing 100 parts of aglycidyl methacrylate copolymer ner as described in Example 1. The maskis baked at a temperature of l 10C for 3 minutes and produces apertureshaving a diameter range of 9 to l mils.

A TV tube is constructed and operated in a television set as describedin Example 1. The tube produces a color picture of acceptable quality.

EXAMPLE 4 A first powder coating composition is obtained by mixingtogether 100 parts by weight of polystyrene (M,. 40,000) with two partscarbon black and one part of the antistatic agent of Example 2. Themixture is processed in the manner described for the first compositionin Example 1. The average particle size produced is 20 microns.

The first composition is sprayed on a mask as described in Example 1.The mask is baked at a temperature of 160C for 3 minutes and theapertures size is reduced to 10 to 11 mils diameter.

A second powder coating composition is prepared by mixing together 100parts by weight of an acrylic copolymer (methylmethacrykite 60 percentand butyl methacrylate 40 percent) M, 5,000, with three parts carbonblack as described for the second composition in Example 1. The secondcomposition is applied to the mask having the first composition thereonas described in Example 1 and the mask is baked at a temperature of l10C for 3 minutes. This action reduces the size of the apertures to 9 to10 mils diameter with no plugging of the apertures noted.

A color picture tube is manufactured and operated as described inExample 1. An acceptable quality picture is received on the tube. Priorto forming the complete tube, the compositions are removed from the maskby the use of acetone.

- EXAMPLE A first powder coating composition is formed by mixing 100parts by weight of polystyrene (M,, 70,000) and two parts carbon black.These two ingredients are mixed together in a twin shell tumbling mixerfor minutes and extruded through a compounding extruder at a temperatureof 200C. The Extrudate is pulverized in a fluid energy mill to passthrough a 270 mesh screen and the resulting powder has an averageparticle size of microns and constitutes the first powder coatingcomposition.

The first composition is applied to a shadow mask in the same manner asdescribed in Example 1 and is baked at a temperature of 180C for 5minutes. The size of the apertures in the mask is reduced to the rangeof 10 to 11 mils.

A second powder coating composition is formed by a mixture of 100 partsby weight of an epoxy resin (Epon 1004) having a molecular weight of1,800 and four parts carbon black. The ingredients are mixed together ina twin shell tumbling mixer for 10 minutes and extruded through acompounding extruder at a temperature of 130C. The extrudate ispulverized in a fluid energy mill to'pass through a 270 mesh screen andthe resulting powder has an average particle size of 15 microns and isthe second powder coating composition.

The second composition is applied to the already coated mask in the samemanner described in Example 1. The coated mask is baked at a temperatureof C for 3 minutes and the size of the apertures is reduced to 9 to 10mils.

A color TV tube is constructed and operated as described in Example 1.An acceptable quality picture is received on the tube. Prior to formingthe complete color tube, the compositions which have been sinteredtogether in the heating steps are removed from the mask by the use ofacetone.

EXAMPLE 6 A first powder coating composition is prepared by mixing partsby weight of a copolymer formed with 80 percent methyl methacrylate and20 percent styrene (M,, 40,000) with two parts carbon black in themanner described for the first powder coating composition in Example 1.The material is applied to a shadow mask as described in Example 1 andbaked at a temperature of C for 3 minutes to produce a final aperturesize in the range of 10 to 11 mils.

A second powder coating composition is prepared by mixing 100 parts byweight of a copolymer containing 5 percent glycidyl methacrylate, 75percent methyl methacrylate, 20 percent ethyl acrylate (IT/l, 5,000) andtwo parts carbon black. These materials are blended as described for thesecond composition in Example and applied to the already coated mask asdescribed in Example 1. The mask'is baked at 100C for 5 m nu es, d thaperwa i is d s tqth an of 9 to 10 mils with no plugging of theapertures observed. A color TV tube is constructed and operated asdescribed in Example 1. A good picture is received thereon.

EXAMPLE 7 A first powder coating composition is prepared by mixing 100parts by weight of an epoxy resin, in this case, Epon 1010 similar tothe epoxy resin previously described but with an n equal to 64 and a M10,000, with four parts carbon black. The materials are mixed togetherin a twin shell tumbling mixer for 10 minutes and extruded throughacompounding extruder at a temperature of C. The extrudate is pulverizedin a fluid energy mill to pass through a 270 mesh screen. The powder soproduced has an average particle size of 20 microns and is the firstpowder coating composition.

The first composition is applied to a shadow mask by an electrostaticspray gun operating at 60 KV and the coated mask is baked at 155C for 3minutes to produce final apertures having a size in the range of 10 to11 mils.

A second powder coating composition is formulated by mixing 100 parts byweight of an epoxy resin, in this case, Epon 1004 having a molecularweight of 1,800, and four parts carbon black. The materials are mixedtogether in a twin shell tumbling mixer for 10 minutes and extrudedthrough a compounding extruder at a temperature of 130C. The extrudateis pulverized in a fluid energy mill to pass through a 270 mesh screen.The powder so produced'is the second powder coating composition and ithas an average particle size of 20 microns.

The second powder coating composition is applied to the coating mask byan electrostatic spray gun operating at 60 KV and the so coated mask isbaked at a temperature of 90C for 3 minutes. The baking sinters thecompositions together and the final size of the apertures is in therange of 9 to 10 mils.

A color picture tube is manufactured as described in Example 1. The tubeis operated and the quality of the picture is acceptable. Thecompositions are removed from the mask by a dichloromethane solvent inorder to utilize the mask in the TV tube.

EXAMPLE 8 A first powder coating composition is prepared by mixing 100parts by weight of an epoxy resin, in this case, Epon ll0, M 10,000,with two parts carbon black. The epoxy resin and carbon black are mixedtogether, mill rolled and pulverized to pass through a 270 mesh screen.After this mixing, five-tenths parts of fumed silica, in this case,Cab-O-Sil M-5, is dry blended with the materials by shaking. Thisproduces a first powder coating composition.

The powder is applied to a shadow mask by an electrostatic spray gunoperating at 60 KV and the coated mask is baked at 160C for 3 minutes toproduce aperture sizes in the range of l0 to l l mils.

A second powder coating composition is formulated by mixing 100 parts byweight of a copolymer formed of 65 percent methyl methacrylate and 35percent ethyl acrylate, (M, 5,000) with three parts carbon black. Thesecond powder coating composition is applied to the previously coatedmask as described in Example and baked at a temperature of 100C for 3minutes to produce apertures in the size range of 9 to mils.

A color TV tube is constructed in the manner described in Example I. Thetube is operated and. the quality of the picture is acceptable. Thematerials are removed from the mask prior to its assembly into the tubeby utilization of acetone.

EXAMPLE 9 A first powder coating composition is prepared by mixing 100parts by weight of cellulose acetate butyrate, in this case, EAB38l-2 ofEastman Chemical Company and two parts carbon black. The materials areprocessed as described in Example 1 to produce a first powder coatingcomposition having average particle size of microns.

The first composition is applied to a shadow mask as described inExample 1 and baked at a temperature of 170C for 3 minutes to produceaperture sizes of 10 to ll mils.

A second powder coating composition is prepared exactly the same manneras described for the second composition of Example 8 and is applied inthe exact manner as described in Example 8 to produce a coated maskhaving apertures with sizes in the range of9 to 10 mils. The shadow maskis used to construct a TV tube as described in Example 8 and a colortube of acceptable quality is produced.

EXAMPLE 10 A first powder coating composition is prepared by mixing 100partsby weight of cellulose acetate buty;

rate, in this case, Eastman Chemical Company half sec ond butyrate, withthree parts carbon black. These materials are processed as described inExample 9 to produce a first powder coating composition having anaverage particle size of 20 microns.

The first composition is applied to a shadow mask as described inExample 9 and baked at a temperature of 155C for 3 minutes. Such actionproduces aperture sizes of 10 to 11 mils in diameter.

A second powder coating composition is prepared in the exact same manneras described for the preparation of the second powder composition ofExample 8. The second powder composition is applied and baked on thealready coated mask as described in Example 8.

The so produced shadow mask is used to construct a TV tube as describedin Example 8. A color television tube of acceptable quality is produced.

EXAMPLE l l A first powder coating composition is prepared by mixing 100parts by weight of a copolymer containing 70 percent methylmethacrylate, 20 percent butyl methacrylate and 10 percent styrene, (M,15,000) and three parts carbon black. The materials are processed asdescribed in Example 1 to produce a first powder coating compositionhaving a particle size of 30 microns.

The first composition is applied to a shadow mask as described inExample 1 and baked at a temperature of 150C for 3 minutes. This actionproduces aperture sizes of 10 to 11 mils.

A second powder coating composition is prepared and applied to thecoated shadow mask in exactly the same manner as described for thesecond powder coating composition in Example 7. The coated mask isemployed in preparing a color TV tube and the color TV tube is operatedas described in Example 1 with an acceptable color picture beinggenerated thereon.

EXAMPLE l2 A first powder coating composition is prepared in the samemanner as described for the preparation of the second powder coatingcomposition described in Example 2. The first powder coating compositionis applied to a shadow mask and baked at a temperature of C for 3minutes. i

A second powder coating composition is prepared and applied to thecoated mask as described for the second powder coating composition ofExample 3. The second composition is baked at a temperature of l 10C for3 minutes.

Because the melt viscosity of these first and secondpowder coatingcompositions are too close together, not enough size reduction can beobtained in the shadow mask apertures and the final size thereof in atwo-step coating process is only 13 to 14 mils. The shadow mask,therefore, is unacceptable for making a color TV tube.

We claim:

1. In the method of making a metallic shadow mask foruse in themanufacture of a color picture tube having a plurality of phosphor colordots placed on the face plate comprising the steps of making saidmetallic shadow mask with a plurality of apertures therein arranged in apredetermined manner, each of said apertures having a predetermined sizelarger than the size of each of said phosphor color dots, reducing thepredetermined size of each of said apertures to a smaller size byapplying a coating, and removing the applied coating from the coatedshadow mask after said plurality of color dots have been placed on saidface plate of said color picture tube to return the size of saidapertures to said predetermined size prior to coating, wherein theimprovement comprises: electrostatically spraying said coating upon saidshadow mask, said coating being a first powder coating compositionhaving an average particle size below 30 microns and comprising asynthetic polymeric material having a molecular weight M1, in the rangeof 10,000 to 100,000, heating said coated shadow mask to a temperatureand for a time sufficient to sinter said coated powder coatingcomposition together but insufficient to induce any substantial flow ofsaid coated powder composition, electrostatically spraying a secondcoating upon said shadow mask, said coating being a second powdercoating composition having an average particle size below 30 microns andcomprising a synthetic polymeric material having a molecular weight (M,,ir1 the range of 1800 to ,000 and 7 heating said coated shadow mask,coated with said first and second coating composition, to a temperatureand for a time sufficient to sinter said second coated polymericmaterial together but insufficient to induce any substantial flow ofeither of said second or said first coating composition. 2. The methodas defined in claim 1 wherein: said first and said second powder coatingcompositions contain a small weight percentage of an antistatic agent.

3. The method as defined in claim 1 wherein: said step of removingcomprises dissolving said first and second coating compositions in asuitable solvent therefor.

4. The method as defined in claim I wherein: said first powder coatingcomposition is principally formed from polymethyl-methacrylate.

5. The method as defined in claim 1 wherein: said first powder coatingcomposition is principally formed from polystyrene.

6. The method as defined in claim 1 wherein: said first powder coatingcomposition is formed principally from glycidyl methacrylate copolymer.

7. The method as defined in claim 1 wherein: said first powder coatingcomposition is principally formed from epoxy resin.

8. The method as defined in claim 1 wherein: said first powder coatingcomposition is principally formed from cellulose acetate butyrate.

9. The method as defined in claim 1 wherein: said second powder coatingcomposition is principally

2. The method as defined in claim 1 wherein: said first and said secondpowder coating compositions contain a small weight percentage of anantistatic agent.
 3. The method as defined in claim 1 wherein: said stepof removing comprises dissolving said first and second coatingcompositions in a suitable solvent therefor.
 4. The method as defined inclaim 1 wherein: said first powder coating composition is principallyformed from polymethyl-methacrylate.
 5. The method as defined in claim 1wherein: said first powder coating composition is principally formedfrom polystyrene.
 6. The method as defined in claim 1 wherein: saidfirst powder coating composition is formed principally from glycidylmethacrylate copolymer.
 7. The method as defined in claim 1 wherein:said first powder coating composition is principally formed from epoxyresin.
 8. The method as defined in claim 1 wherein: said first powdercoating composition is principally formed from cellulose acetatebutyrate.
 9. The method as defined in claim 1 wherein: said secondpowder coating composition is priNcipally formed from an epoxy resin.10. The method as defined in claim 1 wherein: said second powder coatingcomposition is principally formed from a glycidyl methacrylatecopolymer.